Greek media reported that the German embassy and the Goethe Institute will receive additional protection.
Relations between Greece and Europe's effective paymaster Germany have been strained, with Merkel blamed in Athens for imposing austerity at all costs and the Greeks criticised in Berlin for not keeping promises.
On Sunday, government and opposition parties described the chancellor's visit to Greece as a chance to show solidarity and recognition of its efforts.
Merkel's trip to Athens is "an act of recognition for the Greek government which is under great pressure with its reform policy," Foreign Minister Guido Westerwelle said.
The minister, who is from the Free Democratic Party (FDP), a junior partner in the centre-right coalition, told Monday's Bild newspaper that the Greeks had earned "fairness and respect."
For his part, the FDP's parliamentary chief, Rainer Brüderle, told the Welt am Sonntag newspaper that the chancellor's visit was "a clear sign of our solidarity with Greece."
Politicians from the opposition Social Democratic Party (SPD) and Greens struck a similar tone ahead of the visit, Merkel's first to Greece since the eurozone debt crisis ignited nearly three years ago.
Merkel must make it clear that "we are helping out of mutual interest and not as a rich uncle who knows everything better," European Parliament president Martin Schulz, an SPD member, told the Leipziger Volkszeitung.
Late Sunday, German Finance Minister Wolfgang Schäuble, told public television channel ZDF that the point of Merkel's visit was not to weigh in on what the "troika" must decide, referring to Greece's international creditors, the International Monetary Fund, the European Central Bank and the European Union.
The group must approve spending cuts necessary to unblock a €31.5 billion installment from Greece's second EU-IMF rescue package.
"The troika must say if Greece is or isn't fulfilling its obligations," Schäuble said.
The Green party's parliamentary group leader Jürgen Trittin meanwhile urged Merkel in the Welt am Sonntag to make clear to the Greeks that "they can count on European solidarity on the hard path ahead of them."
Merkel's spokesman Steffen Seibert said Friday the message she would take to Athens would be that "we want to help Greece stabilise itself within the eurozone."
"We are doing that by massively contributing to the two aid packages that are supposed to help Greece come out of the crisis," he said, stressing that aid was possible only if Greece stuck to the austerity cuts demanded by international creditors.
Nevertheless, he said Berlin saw greater efforts being made under Prime Minister Antonis Samaras.